Well regarding "TSL" I'm sure you mean TLS, and it's not just an alternative to SSL, it's an evolution to SSL. SSL nowadays is considered insecure and everything is using TLS.
About the SFTP vs FTPS, SFTP is a completely new protocol based on SSH. FTPS is the plain old FTP wrapped in a TLS/SSL socket.
If that is the case please explain the following scenario.
I wrote an editor that had FTP capabilities but it was based on the FTP RFC 913 specification using nothing but plain old sockets.
As a result, many user loved the FTP feature, but they asked for a version that would work over the SFTP protocol.
So I implemented an SFTP version of that editor by using OpenSLL to create a SSL socket. The editor just created a secure socket, but used the exact same FTP RFC 913 specification to talk to the server using that socket.
Ever user who asked for this change reported that it now worked perfectly with their SFTP (not TLS) server.
So how is that possible that my editor could implement the SFTP protocol with just a change to the way the socket was opened?
PS: If you don't believe me, you can trial that exact same SFTP (OpenSSL) editor from the link below and I'm pretty sure you'll find it still works with an SFTP server: http://www.zeusedit.com/download.html
Maybe your customer mistook their SFTP server for an FTPS server? I'm afraid I don't have a Windows machine to test your software on, but if you're curious I encourage you to set up an SSH server like OpenSSH (which provides SFTP as well) and try with that. I'd be really surprised if your FTPS implementation managed to work with it (unless OpenSSH's SFTP implements an FTPS fallback).
